It has been traditional in the art of sheet-fed printing presses to provide systems for supporting freshly printed sheets when transferring the sheets from one printing unit to another or when handling the sheets as they are transferred by a press delivery system from the last printing unit to a sheet delivery stacker. A sheet transfer system comprises a support roller or cylinder disposed between one or more printing units in the press and which functions to receive a freshly printed sheet from one impression cylinder and transfer the sheet to the next printing unit for additional printing. The press delivery conveyor system usually includes chain driven gripper bars which receive the freshly printed sheets from the last impression cylinder of the press and deliver the sheets to the press delivery stacker.
Because the inks used with offset printing presses typically remain wet and tacky for some time, marking and smearing of the freshly printed ink is a concern in all sheet transfer and delivery systems. When transferring a sheet between printing units, marking or smearing of the printed side of the sheet is often caused by a fluttering motion of the sheet as it transfers through a reverse curvilinear path from the impression cylinder to the next transfer cylinder.
Turbulent air movement is caused by a delivery venting system which extracts moisture, volatile vapors and odors released from the freshly printed and/or coated sheets. Such delivery venting systems typically include a hood that is mounted above the delivery sheet stacker through which air is drawn up from the vicinity of the press delivery stacker. The resulting turbulent air flow in the delivery area of the press often causes fluttering motion of the sheets as they are released over the sheet delivery stacker. Moreover, after the grippers release, free fall of the sheet is retarded by the updraft of the delivery venting hood so that the trailing edge portion of a sheet floats momentarily, then contacts the next gripper bar assembly, thus resulting in a sheet jam-up in the delivery stacker.